PHOTOASSISTED PRODUCTION OF HYDROGEN FROM UREA OXIDATION – HYUREA

Nanostructured a-Fe2O3 layers absorbing UV-visible light were synthesised hydrothermally on conductive FTO (Fluorine doped Tin Oxide) glasses and characterised by Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDX) for morphological and elemental analysis. Two methods were used to deposit nickel, a catalyst for urea oxidation, on the photoactive layers: sputtering and photoelectrodeposition (PED). Photocurrents are measured by cyclic voltammetry (CV) in alkaline medium with urea (0.33 mol L-1), under illumination (1000 W m-2). In situ transmittance measurements during the CV measurements provided mechanistic information on the oxidation of urea.
The study of the electrochemical mechanisms of urea oxidation on nickel is complemented by voltammetry and impedance studies in a controlled electroactive species transport regime.
Electrolysis was also carried out for which it was necessary to develop the conditions for the determination of gases and dissolved species. The gases were determined by Gas Chromatography and the ionic species by Ion Chromatography. The material chosen is a polished nickel plate and most of the preliminary tests were carried out in a type H electrochemical cell (about 100 cm3 /compartment).
An electrochemical reactor is under construction and simulation tests of the reactor have been carried out in Comsol.

Nanostructured a-Fe2O3 layers were synthesised hydrothermally on FTO conducting glasses. First, the influence of the synthesis conditions (time, reagent concentrations, doping) was examined in order to select the material with the best performance for the photooxidation of water. Subsequently, two methods were used to deposit nickel, a catalyst for urea oxidation, on the optimised photoactive layers: sputtering and photoelectrodeposition (PED). The photocurrents measured on the a-Fe2O3 / Ni photoanodes have average values of 0.36 mA cm-2 and 0.52 mA cm-2 for the sputtering and PED electrodes, respectively. Using product gas assays, it was shown that urea oxidation occurred on the photocurrent plates. In situ transmittance measurements during the VC measurements provided mechanistic information on urea oxidation, confirming the presence of a chemical reaction coupled to the electrochemical processes.
An electrochemical cell was developed for the electrolysis of urea. Preliminary electrolyses, carried out without photocurrent on polished nickel plates, have shown faradic yields of N2 (target molecule) of the order of 40% in the best case and more than 80% degradation of urea; these electrolyses have, moreover, depending on the operating conditions, led to rather undesirable by-products (nitrites and cyanates), whose concurrent formation seems to be controllable.
An electrochemical reactor is under construction and simulation tests of the reactor have been carried out using Comsol, but the kinetic laws used have not yet allowed the experimental results to be validated. Kinetic studies are therefore underway to produce suitable laws.

The photocurrent values are still low compared to the objectives of the project (5 mA cm-2) and we plan to test silicon photoanodes. With this type of material, the achievable currents are of the order of 20 mA cm-2 under solar illumination. We will therefore prepare Si-Ni photoanodes by depositing nickel either by sputtering or by photoelectrodeposition.
The results of the electrolysis show that yields of the order of 40% are obtained for the target molecule N2. We plan to study the role of a co-element, rhodium, which has the effect of amplifying the electrocatalytic currents. Thus, studies will be carried out to examine the influence of rhodium on the oxidation products, and for this purpose, different methods are envisaged for depositing rhodium (chemical displacement, electrodeposition). We expect an increase in the efficiency of the urea oxidation reaction towards an increase in the amount of N2 produced.
Cyclic voltammetry and impedance studies will complement these measurements by determining the electrocatalytic properties of powdered NiO and NiO-rhodium bimetals in alkalinised urea and synthetic urine solutions. The objectives are to determine the role of rhodium in the amplification of the measured intensities but also the influence of the constituents of the urine as well as the impact of the presence of the various reaction products identified in the solution during the electrolyses.
When the electrolysis conditions (cell, materials, experimental conditions) are completely mastered and the electrochemical reactor is built, we will consider the addition of solar assistance in order to develop the photoelectrochemical reactor targeted in the project.
The voltammetric tests and electrolysis with real urine will be carried out at a later stage due to the variability of its composition.

Oral : Photoelectrochemical Conversion of Urea on FTO/Fe2O3/Ni Photoanodes for the Production of H2, L. Rebiai, C. Cachet-Vivier, D. Muller-Bouvet, S. Azimi, V. Rocher, S. Bastide, 29th ISE Topical Meeting, Mikulov, République Tchèque, 18-21 avril 2021 (On line)

Oral : Quantification of urea electrolysis products in alkaline medium, R. Benyahia, L. Rebiai, L. Latapie, G. Hopsort, K. Serrano, K. Loubiere, T. Tzedakis, S. Azimi, V. Rocher, E. Torralba-Penalver, S. Bastide, 72nd Annual Meeting, Jeju Island, Corée, 29 août-3 septembre 2021 (On line)

Poster : Electro-oxydation photoassistée de l’urée simultanée à la production d’hydrogène, G. Hopsort, L. Latapie, K. Loubière, K. Serrano, T. Tzedakis, C. Cachet-Vivier, R. Benyahia, L. Rebiai, D. Muller, S. Bastide, Journées Nationales de l'Énergie Solaire JNES 2021, 25 au 27 aout 2021, Odeillo, Pyrénées Orientales

Poster : Photoelectrochemical conversion of urea on FTO/Fe2O3/Ni photoanodes, L. Rebiai, D. Muller-Bouvet, Sam Azimi, V. Rocher, C. Cachet-Vivier, S. Bastide, 5ème journée du GDR Solar Fuels, à Saint Jacut de la Mer, France, 27-29/09/2021

Oral : Photoelectrochemical conversion of urea on FTO/Fe2O3/Ni photoanodes, L. Rebiai, Journée Des Doctorants de l’Université Paris Est

Oral flash: Study the performance of different electrocatalysts for urea electro oxidation, S. Akkari. Winter School Grenoble Energy Conversion & Storage Winter School ENGINE 2021. France 16/02/2021

The project HYUREA aims at developing a photoelectrochemical reactor (PER) to depollute urine effluents at low cost by converting urea into a storable fuel, namely H2:
CO(NH2)2 + H2O + solar-h??+ electrochemical-?V -->? CO2 + N2 + 3 H2 (R1)
Comparison of the standard potentials of the systems Urea/CO2, N2, H2 (0.37 V) and Water/O2, H2 (1.23 V) shows that R1 requires an energy threefold lower. The vast untapped, decentralized and renewable resource of urea in urine (60 Mt/year from humans) makes R1 very attractive, whereas most of studies focuses on solar water splitting.
The main scientific challenges of HYUREA are:
(1) to design and study large photoanodes made of nanostructured Fe2O3 + Ni-Metal nanocatalysts (100 cm2) for an efficient solar energy conversion (UV-Vis) with photocurrents above 5 mA/cm2 in presence of urea in urine (0.33 M).
(2) to design, build and optimize an innovative PER integrating the elaborated photoanodes and enabling urea mineralization (~5 L of urine/day.m2, conversion >70%) with H2 production (~100 L/day.m2 at ambient T, P) using both solar light and external applied bias.
(3) to install and test a pilot PER on a building site equipped with selective collection of urines.

To perform this research, HYUREA is structured in four tasks, each leaded by one partner:
T1 – Elaboration of the electrodes - ICMPE
1.1: Photoanodes (ICMPE, LISE). Deposition of Fe2O3 by sputtering or hydrothermal method followed by electrodeposition of Ni-M nanocatalysts. 1.2: Cathodes (ICMPE). Electrodeposition on stainless steel substrates of Ni-Mo nanoparticles. For both subtasks: morphological and structural characterization (XRD, SEM), metal content and composition analysis (ICP-OES and EDX).
T2 – Photo/electrochemical characterization of the electrodes - LISE
2.1: Photoanodes (LISE, ICMPE). (i) A-T-R, I-V characteristics, spectral response, Photo-SECM and IMPS (Intensity Modulated Photocurrent Spectroscopy) to investigate charge separation and transport. (ii) Electrochemical studies with synthetic/real urine to identify possible problems related to this complex matrix. Investigation of the photoanode stability and urea electro-oxidation mechanism pathway under illumination by SECM and EIS techniques. 2.2: Cathodes (LISE, ICMPE). Voltammetric and EIS measurements of Ni-Mo nanoalloys activity towards HER.
T3 – Production of a photoelectrochemical reactor (PER) - LGC
Design and optimization of a solar driven flat plate PER of 100 cm2 assisted by an applied external bias. Key issues: (i) Appropriated PER’s frame, materials and configuration for efficient light management; (ii) optimization of the PER geometry (thickness of compartment(s), suppression of the membrane/system to avoid hydrogen oxidation); (iii) effect of operating parameters (pH, T, flow rate).
T 4 – Urine management and global implementation of the process - SIAAP
4.1: Conditioning steps and storage of urine effluents on site (SIAAP). Physicochemical characterization of real urine after separate collection and its evolution as a function of storage time and conditioning steps (pH). 4.2: Pilot scale implementation (LGC, SIAAP). The PER will be tested at a pilot scale on a site built by SIAAP allowing separate collection and storage of urines. Monitoring of reactants and products (urea, H2, mineral ions or organic contaminants) vs. operating parameters. 4.3: Hydrogen storage (LGC, ICMPE).
Three academic partners (ICMPE, LISE, LGC) and the largest Paris sanitation operator (SIAAP) will work together so as to build and install the PER pilot. ICMPE, LISE and LGC have complementary skills in the synthesis and characterization of metal oxides, bimetallic catalysts for urea oxidation, photo/electro/chemistry, electrochemical engineering and hydrogen storage. SIAAP has analytical means to study waste waters and pilot sites for urine collection. This, together with the human forces involved, will enable a full implementation of the project.